Angle of rotation sensor with an asymmetrically positioned permanent magnet

ABSTRACT

The invention pertains to an angle of rotation sensor, wherein a magnet that is arranged such that it can be turned relative to a sensor that is sensitive to magnetic fields is coupled with a rotatable body, the angle of rotation (α) of which needs to be measured. The invention aims to significantly broaden the measuring range of such an angle of rotation sensor. The invention proposes to utilize an asymmetric magnetic field such that the maximum and the minimum of the magnetic field measured by the probe are spaced apart by an angular range in excess of 270°. Advantageous additional developments pertain to other options for adapting the magnetic field to the desired shape. These options include an air gap that changes over the angle of rotation and the utilization of asymmetric pole shoes.

BACKGROUND OF THE INVENTION

The invention pertains to an angle of rotation sensor, wherein therotational position of a permanent magnet is evaluated by at least oneHall element. In sensors of this type, the rotational position of thebody, the angular position of which needs to be determined, is coupledwith at least one permanent magnet. If the rotational position ischanged, the distribution of the corresponding magnetic field is alsochanged relative to a stationarily arranged Hall element. Consequently,the angular position of the body that is coupled with the permanentmagnet can be determined based on the voltage measured on the Hallelement.

U.S. Pat. No. 4,829,248 discloses a rotational body in which a series ofpermanent magnets is arranged on its outer circumference. If this bodyrotates, the intensity and the direction of the magnetic field thatpenetrates two Hall elements is changed, wherein the Hall elements arestationarily arranged opposite to the rotating body in the vicinity ofthe outer circumference. One disadvantage of this measuring arrangementcan be seen in the fact that it requires a large number of permanentmagnets. In addition, this measuring arrangement is designed merely forevaluating the relative movement between the body surface and the Hallprobe. Consequently, an absolute measurement of the position of the bodywithin a small angular range is not possible with the measuringarrangement disclosed in U.S. Pat. No. 4,829,248. U.S. Pat. No.5,325,005 discloses a synchronous motor, the rotational field of whichis controlled as a function of the rotational position of the armature.In this case, the Hall probes serve as triggers for advancing therotational field.

In known sensors for angles of rotation which operate with Hall probes,a value for the angle of rotation of approximately 180° lies between thepositive and the negative maximum of the measured magnetic flux, andconsequently of the voltage delivered by the Hall probe. If themeasurement is carried out over a range that exceeds 180°, the measuredvalues may be ambiguous. For an angular value that exceeds 180°, adefinitive measurement result can be obtained by utilizing two or moreHall probes. However, this method requires a larger number of Hallprobes, i.e., the expenditure for the corresponding evaluation is alsoincreased. The invention aims to disclose an arrangement that makes itpossible to measure the angle of rotation over an angular range that byfar exceeds 180°, namely with the least possible technical expenditureand, in particular, a small number of Hall probes.

SUMMARY OF THE INVENTION

The invention, in principle, proposes to provide the rotatable magnetwith a yoke that essentially surrounds the magnet in circular fashion,and to design and/or rotationally arrange the magnet (with or withoutpole shoes) within the yoke in such a way that it delivers definitivemeasuring results over a measuring range that by far exceeds 180°. Inthis context, the term yoke refers to a stationary, magneticallyconductive body that surrounds the magnet and forms a path for themagnetic flux emerging from the magnet which has a superior magneticconductivity. In this respect, the yoke acts similarly to the stator ofan electric motor. However, the yoke simultaneously fulfills a shieldingfunction with respect to possible external electromagnetic interferencefields. In this case, the yoke does not necessarily have to be designedcircularly. The yoke may also have a different shape as long as it isable to increase or keep the angular measurement range of the angle ofrotation sensor large. The yoke may, for example, have an ellipticalinner contour. However, it is essential to the invention that allpossible measures which produce the largest possible angular rangebetween the measured maximum and the measured minimum of the magneticflux be combined with one another, wherein the angular range to the nextmaximum is correspondingly shortened within the adjacent measuring rangebecause the original position of the angle of rotation sensor isnaturally reached again after 360°.

Consequently, the measures according to the invention consist, inprinciple, of designing the angle of rotation sensor in an asymmetricfashion such that an angular range is formed in which the maxima lie asfar apart from one another as possible. In a particularly simple measurefor achieving this objective, the magnet is arranged to be offsetrelative to its rotational axis. In addition, the rotational axis of themagnet may also be arranged to be offset relative to the rotational axisof the body, the rotational movement of which needs to be measured.However, this is not necessary. If both axes are aligned with oneanother, a simpler mechanical construction is attained such that themagnet and the body, the rotational movement of which needs to bemeasured, can be arranged on a common axis. The mechanical constructioncan be additionally simplified by aligning the central axis of the yokewith the rotational axis of the magnet which may also be aligned withthe rotational axis of the body to be measured. With respect to themagnet, it can be generally stated that this magnet may be magnetizedthroughout or provided with pole shoes that direct the magnetic flux ofthe core magnet to the yoke via an air gap in suitable fashion withinthe yoke space. In order to maintain the magnetic resistance for themagnetic flux at a minimum, the air gap between the magnet and the yokeor between the pole shoe and the yoke, respectively, should bemaintained as small as possible over the entire angle of rotation. Withrespect to the offset position of the magnet relative to its rotationalaxis, the magnet may be arranged in such a way that its flux within themagnet core extends radially to the contour of the adjacent yoke regionor tangentially thereto. These indications may also refer to therotational axis of the magnet, i.e., the inner magnetic flux eitherextends radially to the rotational axis or tangentially thereto. In thepresent embodiment, a radial orientation of the magnetic flux isproposed. However, if deemed practical with respect to the othercomponents of the angle of rotation sensor according to the invention,it is also possible to choose a tangential internal magnetic flux or todefine an intermediate position for the magnet which lies between theradial and the tangential arrangement.

If the magnet is arranged to be radially offset from its rotational axisand a radial internal magnetic flux is chosen for the magnet core, onerealizes an embodiment which is characterized by a high flux densitywithin the yoke.

A particularly simple mechanical arrangement is defined by a body, thecontour of which is arranged to be circularly symmetrical within theyoke, but causes a highly asymmetric magnetic field to result whichcontributes significantly to improving the desired result.

In a particularly simple design for a magnet that preferably does nothave pole shoes, the magnet has an essentially circular or annular crosssection. If the magnet has an annular cross section, the magnetic fieldis preferably aligned by providing the ring with two radially magnetizedregions which oppose one another, wherein one region extends over a muchlarger angle than the other region. Due to this measure, the desiredasymmetric distribution of the flux over the yoke circumference isachieved with a correspondingly asymmetric distribution of the radiallyextending field within the air gap between the magnet or pole shoe andthe inner contour of the yoke.

Another principle for attaining the desired asymmetry is defined by anarrangement which contains a permanent magnet that may be arrangedcentrally with reference to the rotational axis of the magnet orradially offset thereto. This permanent magnet usually has only onecontinuous magnetization that extends to one side. In order to attain orimprove the desired asymmetry of the magnetic flux or adapt thisasymmetry of the magnetic flux to the corresponding requirements, thepermanent magnet is provided with pole shoes that are preferablyrealized in the shape of sectors of a circle. In order to attain,improve and adapt to the corresponding requirements of the desiredasymmetry, the pole shoes extend along the contour of the yoke over adifferent angular range. Alternatively or additionally thereto, theouter surface of the pole shoe, which is situated opposite to the innercontour of the yoke, may be provided with a different curvature suchthat the width of the air gap between the pole shoe and the innersurface of the yoke changes over the circumferential angle.

The measuring range can, as initially mentioned, be increased byutilizing two or more Hall probes. A particularly sensitive measurementcan be achieved in the case where the Hall element or the Hall elements,which are preferably offset relative to one another by 90°, are insertedinto the yoke in such a way that they intersect the magnetic flux thatextends tangentially at this location. The design of the yoke should, atleast within this region, be chosen such that the entire magnetic fluxintersects the surface of the Hall element. However, it is notabsolutely imperative to radially insert the Hall element into the yoke.In this respect, it is also possible to insert the Hall element or theHall elements in such a way that the magnetic flux flows radiallythrough the Hall element(s), e.g., as described in U.S. Pat. No.5,325,005. This means that the Hall elements are stationarily arrangedin front of the magnetically conductive yoke, preferably in the air gap.

BRIEF DESCRIPTION OF THE DRAWING

Embodiments of the invention are described below with reference to thefigures which show:

FIG. 1 is a plan view of a first embodiment of the invention in whichonly one Hall probe is used;

FIG. 2 is a plan view of a second embodiment similar to the embodimentaccording to FIG. 1, wherein two Hall probes are used;

FIG. 3 is a representation of the output signals of the sensorsaccording to FIG. 2;

FIG. 4 is a plan view of a third embodiment of the invention;

FIG. 5 is a plan view of a particularly advantageous fourth embodimentof the invention;

FIG. 6 is a diagram of the field progression of the embodiment accordingto FIG. 5;

FIG. 7 is a graph depicting the tangential flux within the yoke as afunction of the angular position of the magnet;

FIG. 8 is an elevation view of an embodiment that is comparable to FIG.4; and

FIG. 9 is a perspective view of the embodiment according to theembodiment shown in FIG. 1 with the yoke 5 having the shape of a hollowcylinder with an elliptical inner contour.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 9 is a perspective view of the embodiment according to theembodiment shown in FIG. 1 with the yoke 5 having the shape of a hollowcylinder with an elliptical inner contour.

FIG. 1 shows a top view of a permanent magnet 1 that is realized in theform of a hollow cylinder and contains a first magnetization region 2and a second magnetization region 3. The magnetization direction withinregion 3 is opposite to the magnetization direction within region 2 asindicated by corresponding arrows. This results in the asymmetricmagnetic field to be attained in accordance with the present invention.As mentioned previously, the magnet is realized in the form of apermanent magnet. However, this is not absolutely imperative for theinvention. If so required, a magnet that is magnetized by means of adirect current may be used instead of a permanent magnet. However, it isessential that an asymmetric magnetization results, which maintains thedefinitive measuring range of the angle of rotation sensor according tothe invention at the desired size.

The permanent magnet 1 can be turned about the rotational axis that isindicated by D and extends perpendicular to the plane of projection,i.e., the axis D represents the central axis of the hollow cylinder 1.The permanent magnet 1 is surrounded by a magnetically conductive yoke 5in essentially a circular fashion, wherein said yoke also has the shapeof a hollow cylinder. One Hall probe 7 is radially inserted into theyoke 5. An air gap 6 that represents a magnetic resistance is situatedbetween the outer contour of the permanent magnet 1 and the innercontour of the yoke 5. If so required, the yoke 5 may also be providedwith an air gap 8, the shape and position of which results from thedesired development of the magnetic field, and the precise position ofwhich may have to be determined by experiment.

A person skilled in the art is easily able to ascertain that theconfiguration shown in FIG. 1 results in an asymmetric magnetic field.In this case, the field lines of the magnetic field extend, for example,from the second region 3 into the yoke 5 via the air gap 6, namely inthe direction of the magnetization arrows N, wherein said field linessubsequently extend to the point at which they emerge from the yoke in afashion not shown. The magnetic field ultimately emerges from the yoke 5and extends into the air gap 6 approximately on the opposite side of theyoke, after which it extends from the air gap into the first region 2 inthe direction of the arrows S. Depending on the shaping of the magneticfield, which depends on the width of the yoke gap 8 and the air gap 6 aswell as on the extent of the magnetization regions of the permanentmagnet 1 and on other parameters, the embodiment according to FIG. 1makes it possible to measure an angular range that is by far greaterthan the conventional angular range of 180°.

In FIG. 3, the curve 10 represents the possible progression of themagnetic flux and consequently also the voltage on the Hall probe overthe angular range. In this case, it can be determined (if the angle a iscorrespondingly assumed to have a zero value) that, over a range of270°, the magnetic flux penetrating the Hall probes 7 continuouslydecreases as the angular value increases, and finally becomesincreasingly negative. This results in a voltage value that continuouslychanges over an angular range of nearly 270° at the output of the Hallprobe, i.e., the measured voltage value clearly defines a definitiveangular value within this region. If a measuring range of only 270° isdefined, the rotational position of the magnet and consequently therotational position of the body, the angle of rotation of which needs tobe measured, can be directly measured based on the aforementionedvoltage value. If one takes into consideration the possibility of alsomeasuring the voltage change as a function of the change of the angle ofrotation, it can also be determined in which curve section themeasurement is being made based on the different slopes of the curvesections 11 and 12, namely by utilizing the differential of the voltageor the slope of the voltage curve for determining the curve section inwhich the measurement is being made. This means that a measuring rangeof nearly 360° can be evaluated under these conditions.

FIG. 2 shows a second embodiment that merely differs from the embodimentaccording to FIG. 1 due to the fact that a second element 14, which issensitive to magnetic fields (i.e., preferably a Hall probe), isinserted into the yoke 5 with an angular offset of 90°. Due to thismeasure, it is possible to obtain the curve 13 in FIG. 3 which, undercorresponding circumstances, can approximately correspond to the curve10 offset, however, by 90°. If an angular range of only 270° isobserved, it can be determined that the curve 13 increases steeply inlinear fashion within a measuring range of 90°, wherein the positiveslope is significantly greater than the negative slope of the curvesection 11 of the curve 10. This effect can be utilized for carrying outmeasurements with higher sensitivity in a measuring range of 90° withinthe total measuring range of 270°, with the aid of the second Hall probe14. In other words, this arrangement makes it possible to carry out amore precise measurement.

FIG. 4 shows a third embodiment, in which the yoke 5 is realizedidentically to that of the first embodiment according to FIG. 1. Theblock-shaped permanent magnet 16 can also be turned about a rotationalaxis D in this case, as indicated by the directional arrow R. Theblock-shaped magnet 16 is magnetized only in one direction. Asymmetricmagnetization is attained due to the fact that two pole shoes 18, 19that have different shapes are attached to the block magnet 16. In thisembodiment, the pole shoes essentially have the shape of sectors of acircle, wherein the first pole shoe 18 extends over a smaller sectorthan the second pole shoe 19. The field distribution is approximatelyanalogous to the field distribution of embodiments 1 and 2, i.e., it canbe expected to obtain the curve 10 shown in FIG. 3.

FIG. 4 shows another option for promoting and influencing the asymmetryof the magnetic field by changing the clearance width of the air gap 6over the angular range. This can be attained by providing the pole shoe19 with a different curvature than that of the inner edge of the yoke 5,e.g., as shown in connection with the first pole shoe 18. However, it isalso possible to shift the rotational axis D of the magnet 16 andconsequently of the pole shoes 18, 19 relative to the central axis ofthe yoke 5 and pole shoe, i.e., to arrange the magnet 16 eccentrically.This is indicated in connection with the second pole shoe 19. Bothoptions can be used individually or in combination as shown in FIG. 4.

In FIG. 4, and as discussed previously, the width of the air gap 6 canbe adjusted to support the desired asymmetry of the magnetic flux. Thisfeature is also shown in FIG. 9, where the yoke 5 is shown as a hollowcylinder with an inner elliptical contour. As in FIG. 1, the magnet 1rotates in either direction indicated by the arrow R around therotational axis D.

FIG. 5 shows a fourth embodiment which should be particularly emphasizedbecause it inexpensively realizes the desired asymmetric field with asimple design. The unit consisting of the permanent magnet 20 and thepole shoe 21 rotates about the rotational axis D. Although this unit 22is arranged symmetrically with reference to the rotational axis D andthe yoke 5, this does not apply to the magnetic field, which has thedesired asymmetric shape. The resulting field distribution isillustrated in FIG. 6 and shows that the flux density within the yoke 5as well as the flux density within the air gap 6 changes continuouslyover the angular range. Since the flux decreases continuously over thecross section of the yoke 5 in both yoke halves between the region 25(in FIG. 5 +90°) and the region 26 (in FIG. 5 −90°), but themagnetization direction in the region 26 simultaneously changes whilethe rotating direction is preserved, an intense field which extends inthe positive direction results in the clockwise direction (see FIG. 6)beginning at +90 (region 25). This field becomes continuously weakeruntil it is ultimately reversed at −90° and continuously increases up tothe region 25 (i.e., up to +90°) in the opposite direction.

FIG. 7 shows the progression of the magnetic flux over the circumferenceof the yoke 5. It can be determined that a progression which changescontinuously in the same direction is attained over 8/10 of thecircumference, i.e., over approximately 290°. It can be expected thatthe measuring range can be additionally increased by utilizingadditional measures described in this publication. In FIG. 7, the valueof the measuring unit is E, which can be interpreted as the magneticflux or as the voltage on the respective Hall probe, and this is plottedas a function of the circumference U.

FIG. 8 shows a fourth embodiment which is realized in very similarfashion to the third embodiment of the invention shown in FIG. 4. Theessential difference between FIG. 8 and FIG. 4 can be seen in the factthat the clearance width of the air gap 6 above both pole shoes is madeidentically large, i.e., a very small air gap is produced and thecurvature of the outer contour of the pole shoes 18, 19 is essentiallyidentical to the inner contour of the yoke 5. By contrast with FIG. 4,the magnetic field caused by the flux of the magnet 16 is shown in FIG.8. One can ascertain that the magnetic flux through the cross section ofthe yoke and in the air gap 6 changes continuously over a very largeangular range. Practical experiments have demonstrated that thedependence of the magnetic flux on the circumference is very linear overthis angular range, i.e., calibration of the sensor for rotationalangles becomes much simpler. According to the invention, it is notabsolutely imperative to use a Hall probe for measuring the magneticfield. Other probes which are suitable for measuring magnetic fields mayalso be used for the present invention. Since the magnetic flux in theair gap changes continuously over a very large angular range in thepresent embodiment, it may, as described above in connection with U.S.Pat. No. 5,325,005, be practical under certain circumstances to insertthe probe into the air gap between the permanent magnet and the yoke 5or the pole shoe(s) and the yoke 5. However, a diminished magnetic fluxwhich intersects the magnetic field sensor (Hall probe) can be expectedin this embodiment of the invention. Since the magnetic flux within theyoke 5 extends perpendicular to the magnetic flux within the air gap 6,the sensor that is sensitive to magnetic fields needs also to becorrespondingly aligned.

Consequently, the invention can be summarized as described below:

Conventional rotational sensors which operate on the basis of magneticfields usually have a maximum measuring range of 180°. This is usuallydefined by the symmetry of the magnetic circuit. In this type ofmagnetic sensor, this symmetry is, in particular, defined by themagnetic flux through the magnet and the pole shoes. In the presentarrangement, the symmetry is eliminated by an appropriate magnet. Anisotropic magnet that was suitably magnetized may, for example, beutilized for this purpose. Alternatively, this magnet may also becomposed of several components, i.e., of a magnet and two correspondingpole shoes.

The sensor according to FIG. 5 utilizes a corresponding magnetic circuitwhich allows a definitive measuring range of up to 330°. FIG. 5 showsthe principle of the magnetic field sensor. This sensor consists of arotating magnet that is arranged eccentrically. The magnet arranged onthe rotating shaft is surrounded by a cylindrical pole shoe. A magneticfield sensor now measures the tangential component of the magnetic fieldin the sensor, i.e., the magnetic flux in the cylindrical pole shoe. Inorder to measure the tangential flux, one or more magnetic field sensorsis/are inserted perpendicular to the circumference of the outer poleshoe.

The advantage of this arrangement can be seen in the superiorutilization of the magnetic flux through the outer pole shoe whichallows the utilization of relatively small magnets. Another advantage ofthis sensor is that only simple and inexpensive magnet shapes arerequired. Only the relative angle between the outer pole shoe with themagnetic field sensor and the inner magnet on the rotational axis isrelevant for the function of the sensor, i.e., either the outer poleshoe or the inner magnet rotates.

What is claimed is:
 1. An angle of rotation sensor, wherein at least one permanent magnet is rotatable with the rotation of a rotatable body about a rotational axis, an angle of rotation of which needs to be measured, wherein at least one Hall probe is arranged outside of the rotational axis, and the voltage of said Hall probe which is fed to an evaluation unit changes as a function of the angle of rotation of a magnetic field of the magnet, characterized in that the magnetic field of the magnet extends between the poles of the magnet through a yoke that surrounds the magnet, and in that the permanent magnet is arranged such that a width of an air gap between an outside edge of the magnet and the yoke varies over the outside edge of the magnet, and the Hall probe detects a magnetic flux that continuously changes in one direction over an angular range of at least 250° during rotation of the rotatable body.
 2. An angle of rotation sensor according to claim 1, characterized in that the magnet is eccentrically arranged with respect to a central axis of the yoke.
 3. An angle of rotation sensor according to claim 1, characterized in that the yoke has the shape of a hollow cylinder with an elliptical inner contour, the longitudinal axis of the cylinder aligned with the rotational axis.
 4. An angle of rotation sensor according to claim 1, characterized in that the permanent magnet is provided with a pole shoe, and the pole shoe is realized mirror-symmetrically with reference to a plane that extends transverse to the flux direction of the permanent magnet and includes the rotational axis.
 5. An angle of rotation sensor according to claim 4, characterized in that the pole shoe essentially has the shape of a circular cylinder, and a radially oriented recess that accommodates the magnet, the magnetic flux of which is also oriented in the radial direction, is arranged in the outer surface of the pole shoe.
 6. An angle of rotation sensor according to claim 1, characterized in that the permanent magnet is provided with a pole shoe, an axis of the pole shoe aligned with the central axis of the yoke.
 7. An angle of rotation sensor according to claim 1, characterized in that the permanent magnet has the shape of a cylinder or hollow cylinder, which has a circular cross section.
 8. An angle of rotation sensor according to claim 7, characterized in that the permanent magnet is in the form of a hollow cylinder having a magnetization that extends radially within the magnet body, wherein the magnetization that is oriented in the direction towards the rotational axis extends over an angular range that differs from that of the oppositely oriented magnetization.
 9. An angle of rotation sensor according to claim 1, characterized in that at least one Hall probe is radially inserted into the yoke.
 10. An angle of rotation sensor according to claim 1, characterized in that at least one probe is arranged tangentially to the inner surface of the yoke in an air gap situated between the yoke and one of the magnet and the pole shoes.
 11. An angle of rotation sensor according to claim 1 wherein the permanent magnet is a block-shaped magnet.
 12. An angle of rotation sensor according to claim 11 wherein the block-shaped magnet is mounted in a recess provided in an annular pole shoe.
 13. An angle of rotation sensor according to claim 11 wherein the block-shaped magnet is provided with radially extending pole shoes that are in the shape of sectors of a circle.
 14. An angle of rotation sensor according to claim 1 wherein the permanent magnet is provided with radially extending pole shoes that are in the shape of sectors of a circle.
 15. An angle of rotation sensor according to claim 14, characterized in that the sectors of the pole shoes extend over different angular ranges.
 16. An angle of rotation sensor according to claim 14, characterized in that a width of an air gap between an outer edge of each pole shoe and the yoke changes over the outer edge of each respective pole shoe.
 17. An angle of rotation sensor, wherein at least one permanent magnet is rotatable with the rotation of a rotatable body about a rotational axis, an angle of rotation of which needs to be measured, wherein at least one Hall probe is arranged outside of the rotational axis, and the voltage of said Hall probe which is fed to an evaluation unit changes as a function of the angle of rotation of a magnetic field of the magnet, characterized in that the magnetic field of the magnet extends between the poles of the magnet through a yoke that surrounds the magnet, and the magnet which is in the form of a permanent magnet is provided with radially extending pole shoes that are in the shape of sectors of a circle.
 18. An angle of rotation sensor according to claim 17, characterized in that the sectors of the pole shoes extend over different angular ranges.
 19. An angle of rotation sensor according to claim 17 wherein the permanent magnet is a block-shaped magnet.
 20. An angle of rotation sensor according to claim 17, characterized in that a width of an air gap between an outer edge of each pole shoe and the yoke changes over the outer edge of each respective pole shoe. 